In recent years the demand for new graphite materials with increased performance in many domains has created a need for new production technologies. For example, the development of graphite for anodes in Li-ion batteries and for use in coating dispersions has seen increased attention in this field.
Amongst these technologies, the grinding of graphite in ball mills has generally been described in the literature. Grinding in ball mills has been performed in both dry and liquid environments to decrease the particle size distribution of graphite down to micron or nano dimensions. Ball mill grinding in liquid medium is usually performed to produce colloidal dispersions. However, the mechanical treatment in a ball mill is typically not suitable to produce anisometric HOGA-like graphite as described in the present invention. Moreover, ball-milled graphite usually shows low electrical resistivity in the cathode.
Byoung et. al. (Kim, Byoung G.; Choi, Sang K.; Chung, Hun S.; Lee, Jae J.; Saito, F. Mining and Materials, Korea Institute of Geology, Daejon, Yoosung-ku, S. Korea. Powder Technology 2002, 126(1), 22-27) describe the grinding of graphite in a low-pressure attrition system. However, this treatment was performed in a dry attrition mill at reduced pressure and elevated temperatures and caused an unspecific reduction of the particle size down to nanometer dimensions. Due to the impact force of the dry attrition, the particles are delaminated and broken indistinctly into smaller parts, while in the HOGA process of the present invention the attrition, which is carried out in liquid medium, is mainly generating delamination. An extremely anisometric form of graphite like the HOGA graphite in the micrometer dimension has not been described by Byoung et. al.
Tsuji et al. (Tsuji, Nobuhiro; Sugimoto, Hisanori. (Nippon Graphite Industries Co., Ltd., Japan), U.S. Pat. Appl. No. 2006046146) mention a process to produce non-exfoliated graphite by peeling-off the graphite layers to produce extremely flaky graphite powders for alkaline battery cathodes. However, the process to peel-off the graphite layers is only described in a very unspecific manner and neither the use of a liquid medium nor an agglomeration of the particles is mentioned by the inventors. The resulting graphite product shows product properties that are clearly distinguishable from those of the HOGA graphite provided herein, in particular with regard to the surface properties. Moreover, the density and specific surface properties of the graphite materials described by Tsuji are shifted during the manufacturing process to decreased values, in contrast to the increase of those parameters during the manufacture of HOGA graphite as described in the present application.
Miura et al. (U.S. 2006/0147796 assigned to Nissan Motor Co., Ltd.) describe a process for making a ground positive electrode active material selected from manganese composite oxides, nickel composite oxides, and cobalt composite oxides. The essence of this invention appears to be the size reduction of the positive electrode active material with different mills such as a vibratory mill, ball mill or sand-mill, followed by the mixing with a conductive additive. Unlike in the present invention, no distinction is made between the effects achieved when different mills are used to prepare the ground material. Furthermore, the only mention of a carbon based material is for use as a conductivity enhancement additive (which has not undergone any form of dry or wet grinding prior to being used). Accordingly, HOGA graphite is neither described, prepared nor used in US 2006/0147796.
Graphite Preparation and Properties
The chemical structure of graphite single crystals is stacked layers of six-membered rings of carbon atoms. The graphite layers are bound together by weak van-der-Waals forces. The interlayer distance between these graphite layers ideally is 0.3353 nm. The hexagonally structured graphite phase, the thermodynamically stable polymorph, shows a stacking sequence of ABAB. Also, a rhombohedral stacking sequence of ABCA is found. Depending on the amount and dispersion of the rhombohedral stackings in the graphite crystal, they can be considered either as isolated rhombohedral phases or as stacking defects of the hexagonal structure. These rhombohedral stacking defects in the hexagonal structure are created by mechanical treatment of the graphite material (graphite milling). Electrical and thermal conductivity within the graphene layers are about 3 orders of magnitude higher than perpendicular to the graphene layers leading to a strong anisotropy of the electrical and thermal conductivity in the graphite crystal.
Usually graphite powders contain polycrystalline particles, i.e. graphite particles contain one or more single crystals which are grown together. Graphite particles have a platelet or flaky shape. Depending on the graphite type, these single crystals are more or less randomly oriented in the particle. The degree of alignment or random orientation gives the mosaicity of the graphite particles, which is a parameter used to describe the graphite texture. The graphite texture is one of the main parameters used to distinguish individual graphite materials and their properties.
Several graphite applications require graphite materials containing particles with high aspect ratio, i.e. with anisometric, flaky or needle-shaped particles. Graphite materials with anisometric particles show low apparent densities. Used as conductive components in electrical conductive masses, graphite materials show percolation thresholds at lower concentration the lower the apparent density is, i.e. anisometric graphites deliver low resistivities at low concentrations because of the higher volume of carbon at the same weight fraction. In addition, in the case of graphite materials with the same apparent density, the graphite materials with the higher aspect ratio (higher anisometric particle shape) exhibit the percolation at lower carbon concentration. The ideal graphite conductive additive in electrochemical electrodes has particles with high aspect ratio, in which large single crystal domains are oriented preferentially along the particle platelet plane combined with a low apparent density or, in other words, a high void volume.
Due to the anisotropy of the graphite structure and texture, mechanical treatments like grinding processes can influence the particle shape. The energy which is required to separate graphite single crystals of a particle and to cleave the graphite single crystals along the van-der-Waals layers is lower compared to the energy which is needed to cut a graphite single crystal perpendicular to the single crystals. Conventionally applied grinding processes like ball milling, air jet milling and mechanical milling techniques usually have a relatively high energy impact on the graphite materials. Thus, the grinding process is less specific for the resulting particle shape. These grinding techniques apply shear forces combined with shock forces with high energy impact to decrease particle size. Usually, they cleave the graphite particles and the graphite single crystals parallel and perpendicular to the xy-plane.
Accordingly, it is an object of the invention to provide novel graphite powders having superior properties compared to powders of the prior art. It is another object of the invention to also provide suitable processes for making such graphite powders.